Electronic Structure and Properties of Poly- and Oligoazulenes

Abstract

One of the most important research interests in the field of organic photovoltaic devices (OPVs) is the development of new materials which can serve as light-absorbing electron donors and hole-conducting (ptype) semiconductors. In this context, 1,3-polyazulenes were synthesized chemically and electrochemically. Their spectroscopic and electrochemical properties are compared with those of the 1,3-oligoazulenes Az 1 -Az 6 . The UV-vis spectra of the neutral azulenes Az 1 -Az 6 show a linear correlation between the lowest absorption maximum and the inverse chain length 1/n leading to a band gap of E g ) 1.90 eV for infinite chain length. Derived from this correlation the effective conjugation length of chemically synthesized polyazulene is only about 10. By an alternative approach, a band gap of E g ) 1.46 eV was determined. Depending on the applied potential the oligomers Az 2 -Az 6 undergo up to two reversible oxidation processes or further polymerization which results in the formation of polymer films at the electrode. The potentiodynamic oxidation of chemically synthesized polyazulene leads to electrocrystallization at the electrode, whereas films of polyazulenes are obtained directly upon oxidation of Az 1 -Az 6 . Chemically and electrochemically generated polyazulenes adsorbed on Pt show similar electrochemical behavior upon positive doping. The spectroelectrochemical investigations in combination with density functional theory (DFT) calculations lead to the conclusion that polyazulene can be oxidized up to a doping level of one charge per three or four azulene units. At this stage polarons or polaron pairs are formed (depending on the doping level) but not bipolarons. At higher doping levels the polymers start to decompose.

Electrochemical preparation of oligo(azulene) on nanoporous TiO2 and characterization of the composite layer

“…The absorption maximum of neutral PAz Fig. 7 The ex situ FTIR spectra of the nanoporous OAz-TiO 2 layer and of the PAz film using a spectrum of ITO/TiO 2 electrode and ITO, respectively, as reference has also been reported by other groups [20][21][22][23][24][25][26][27] to be around 420 nm for both chemically and electrochemically synthesized PAz films. As the absorption maximum of OAz in the composite structure is located at 63 nm higher wavelength it can be concluded that delocalization of the p electrons in OAz in nanoporous TiO 2 is increased.…”

“…This band is due to absorption of Az monomer, which also causes its blue color, and is related to the small band gap of Az [20]. According to the method used by Daub et al [20] the mean effective conjugation length of the OAz in the nanoporous TiO 2 layer can be estimated to 2.2 monomer units and the band gap E g = 2.0 eV. This approximation is based on the linear correlation between the energy of the lowest absorption maximum and the inverse chain length of chemically synthesized oligomers of different length.…”

“…Moreover, a soluble polymer is not required and monomers without any solubilizing side chains can be used. The structure of PAz is based on fused-rings, which is expected to have a reducing effect on the band gap [19,20]. In order to cover most of the solar spectrum, ECPs with an appropriately low band gap are highly sought [20].…”

“…The structure of PAz is based on fused-rings, which is expected to have a reducing effect on the band gap [19,20]. In order to cover most of the solar spectrum, ECPs with an appropriately low band gap are highly sought [20]. The absorbance of PAz in the UV and visible spectral regions is also rather broad [20][21][22][23][24][25][26] making PAz an attractive material for applications in solar cells.…”

“…In order to cover most of the solar spectrum, ECPs with an appropriately low band gap are highly sought [20]. The absorbance of PAz in the UV and visible spectral regions is also rather broad [20][21][22][23][24][25][26] making PAz an attractive material for applications in solar cells. PAz has also been shown to have electron-donating properties [21,[28][29][30][31][32].…”